Conventionally, there have been known hard water softening apparatuses including an ion exchange resin bed as disclosed in the following Patent Document 1. In this kind of hard water softening apparatus, when raw water passes an ion exchange resin bed, hardness components (i.e., calcium ions and magnesium ions) contained in the raw water are exchanged with sodium ions of the ion exchange resin bed. Thus, the hardness components in the raw water are adsorbed to be removed in the ion exchange resin bed, so that it is possible to soften the raw water.
The hard water softening apparatus is an apparatus that exchange sodium ions bonded to the ion exchange resin bed with the hardness components contained in the raw water, and therefore there is a limit to the removal of the hardness components. Therefore, before reaching the limit, exchange capacity is recovered by passing regenerant through the ion exchange resin bed (salt water in the case of the hard water softening apparatus). This is referred to as regeneration of an ion exchange resin bed.
In the hard water softening apparatus, a flow passage is changed according to each of processes such as a water passing process of softening raw water, and a regeneration process of regenerating an ion exchange resin bed. To do the above, a flow passage control valve is provided on the upper part of a pressure tank that houses the ion exchange resin bed, so that the flow passage according to each process is defined by this flow passage control valve. Such a configuration is not limited to the hard water softening apparatus, but is similar to other ion exchange apparatuses including an ion exchange resin bed. That is, the flow passage control valve is also used in an ion exchange apparatus that performs regeneration by passing regenerant through an ion exchange resin bed while passing raw water through the ion exchange resin bed.
As such a flow passage control valve, there has been known a valve disclosed in the following Patent Document 2. This valve (1) includes a valve housing (4) formed with a fluid flow passage (7), and a first port (5) and a second port (6) that serve as doorways to the fluid flow passage (7), and is formed with a valve seat (8) in the middle of the fluid flow passage (7). While a valve body (11) that opens/closes a valve hole (9) of the valve seat (8) is urged to the valve seat (8) by a spring (30), the valve body (11) can be pressed back against this urging force by a valve stem (16).
At the lower part of the valve body (11), a diaphragm-like pressure receiving body (22) is held by a retainer (21), and the spring (30) is provided in the lower part of this retainer (21). In the lower part of the housing (4), a back pressure chamber (28) divided from the fluid flow passage (7) by the pressure receiving body (22) is formed. This back pressure chamber (28) is ordinarily communicated with the first port (5) by a pressure transmission passage (32) formed in the valve body (11), and a through passage (33) formed in the retainer (21).
In a closed state where the valve body (11) is in contact with the valve seat (8), pressure on a side close to the first port (5) is transmitted to the back pressure chamber (28), so that fluid pressure in a valve opening direction and fluid pressure in a valve closing direction applied to the valve body (11) are balanced. Consequently, the urging force of the spring (30) does not need to be increased, and driving force required for opening the valve can be reduced.
Patent Document 1: Japanese Patent Laid-open Publication No. 2008-55392
Patent Document 2: Japanese Patent Laid-open Publication No. 2007-78092